Test to Catch Pot-Intoxicated Drivers Remains Elusive

(CN) – Even as more states legalize recreational and medical marijuana use, law enforcement officials lack a standardized test for measuring cognitive and behavioral impairment from cannabis.

Writing Thursday in the journal Trends in Molecular Medicine, the scientists note differences in gender, amount and method of cannabis use, body-fat percentage and the type of product consumed all affect an individual’s performance on field sobriety tests tailored specifically to marijuana.

Training officers to identify behavioral signs of impairment and establishing a test that accounts for these variables will be critical to maintaining road safety and ensuring that only impaired drivers are arrested.

“There is no one blood or oral fluid concentration that can differentiate impaired and not impaired,” said Marilyn Huestis, who has spent more than 20 years directing cannabinoid-related research projects at the National Institute on Drug Abuse.

“It’s not like we need to say, ‘Oh, let’s do some more research and give you an answer.’ We already know. We’ve done the research.”

One issue with testing for marijuana impairment is how quickly delta-9-tetrahydrocannabinol, or THC – the primary psychoactive agent in marijuana – leaves a user’s bloodstream. Previous research by Huestis has shown that while THC can take six to eight hours to leave an occasional user’s bloodstream, blood-THC concentrations can reach nearly zero within 2.5 hours.

In the United States, it can take 1.4 to 4 hours after a traffic stop or accident to administer a blood test.

“If someone is driving impaired, by the time you get their blood sample, you’ve lost 90 percent or more of the drug,” Huestis said. “So, we have to change what we do at the roadside.”

Chronic marijuana users, such as those who consume it for medical reasons, also present a challenge.

THC accumulates in the body’s tissues and releases slowly, meaning that long-term, daily users can test positive for cannabis after more than 30 days of abstinence. Psychomotor impairment – of or relating to motor action that originates from mental activity – can be observed three weeks after last consumption.

“You want people to be taking medicinal cannabinoids and now you know that their driving is going to be impacted,” said Huestis. “So how do you handle that problem?”

Huestis does not support a legal driving limit for marijuana like what’s used for determining blood-alcohol concentrations. Instead, she advocates for less invasive biological-marker tests that could be performed immediately at the roadside to confirm the presence of a cannabinoid.

Recent research has identified new urine and blood markers, and saliva and breath markers are being developed.

These new markers and tests could also help in treating drug dependence, in determining appropriate dosing for medical marijuana, and for monitoring women who seek to stop using cannabinoids while pregnant.

Huestis, who also owns a toxicology consulting company with co-author Michael Smith, is not opposed to cannabis legalization. However, she does want to ensure that marijuana’s legal status and growing cultural acceptance do not make society overlook its risks.

“Cannabis probably is less dangerous to use than alcohol,” she said. “There’s less morbidity and mortality associated with it, but there’s still a lot of problems.

“And we as a public are not recognizing this.”

Recreational marijuana use is currently legal in nine states and the District of Columbia. Medical use of cannabis is legal in 29 states.